Many hydrocarbon feedstocks contain relatively high concentrations of straight chain and slightly branched chain aliphatic compounds having between 8 and 40 carbon atoms. These compounds tend to form solid waxes upon cooling of the hydrocarbon feedstock. The temperature at which the hydrocarbon oil will not flow is commonly referred to as the “pour point.” The wax forming compounds are generally removed or converted through distillation or hydrotreating processes such as hydrocracking and hydroisomerization. In hydrocracking, high-molecular weight hydrocarbon components are cracked in the presence of hydrogen to lower-molecular weight components. In lubricant base oil dewaxing and/or diesel dewaxing, hydrocracking reactions reduce the waxy content of the feedstock, but can lead to a loss of yield through the production of lower molecular weight hydrocarbons such as middle distillates and even lighter C4-products. Hydroisomerization is another approach to reduce the wax content of feedstocks while minimizing the loss in yield due to the formation of highly cracked low molecular weight products. Hydroisomerization converts aliphatic, unbranched paraffinic hydrocarbons to iso-paraffins and cyclic species which do not easily form waxes.
Hydroisomerization is well known in lubricant base oil dewaxing processes. For example, U.S. Pat. No. 4,222,543 and U.S. Pat. No. 4,814,543 disclose and claim the use of constrained intermediate pore molecular sieves for lube dewaxing. U.S. Pat. No. 4,283,271 and U.S. Pat. No. 4,283,272 claim the use of these catalysts for dewaxing hydrocrackates in energy efficient configurations. Also directed to dewaxing with constrained intermediate pore molecular sieves are U.S. Pat. No. 5,135,638, U.S. Pat. No. 5,246,566 and U.S. Pat. No. 5,282,958. U.S. Pat. No. 4,347,121 claims catalytic dewaxing of hydrocrackates containing less than 10 ppm nitrogen with a hydrofinishing step upstream of the dewaxing catalyst. Important considerations in an efficient dewaxing process include the minimization of catalyst aging and the maximization of yield, particularly with respect to lubricant base oil dewaxing.
Various processes have been tried to minimize catalyst aging. For example, U.S. Pat. No. 5,456,820 discloses a process in which a lube boiling range feedstock is catalytically dewaxed in the presence of hydrogen over a catalyst comprising an intermediate pore zeolite in the decationized form. Catalyst cycle length was found to be improved by optimizing the sequencing of various solvent extracted feedstocks. Multi-layered catalyst systems have also been described as ways to minimize dewaxing catalyst aging. U.S. Pat. No. 5,951,848 and WO 98/02503 disclose the use of a two catalyst system comprising a hydrotreating catalyst and a dewaxing catalyst. The hydrotreating catalyst layer can also be referred to as a “guard bed” or “guard layer”. The aging of the dewaxing catalyst is slowed due to the presence of the hydrotreating catalyst layer or guard layer which protects the dewaxing catalyst from contact with highly aromatic feedstocks which would deactivate the dewaxing catalyst. U.S. Pat. No. 4,749,467 discloses a method for extending dewaxing catalyst cycle length by employing the combination of low space velocity and a high acidity intermediate pore zeolite. The high acid activity and low space velocity reduce the start-of-cycle temperature. Because catalyst deactivation reactions are more temperature sensitive than are dewaxing reactions, low operating temperatures reduce the catalyst aging rate.
While the importance of reducing aging of the dewaxing catalyst has been known and practiced through the use of a multi-layered catalyst system or a guard bed system, as outlined above, it was surprisingly found that conventional guard bed catalyst systems led to a loss in yield of the base oil product in lubricant dewaxing due to the formation of middle distillate and low molecular weight products when the process was run at temperatures over about 600° F. Minimizing yield loss is of particular economic importance in lubricant base oil production. Thus, a lubricant oil dewaxing process in which the dewaxing catalyst is protected from aging and wherein lubricant base oil yield loss is minimized at temperatures over about 600° F. is highly desirable. Surprisingly, it was found that by tightly controlling the acidity of a hydrotreating catalyst upstream of a dewaxing catalyst, lubricant oil yield could be maintained over a wide temperature range for a target pour point.